1. Field of the Invention
The present invention relates to the field of rotary spade drills, and in particular to a rotary spade drill arrangement with improvement features which greatly extend the life of a rotary spade drill of the type having a blade with transverse cutting edges extending from a central portion of the drill tip radially outwardly.
2. Brief Description of the Prior Art
Drills adapted to bore through rock are well known and documented in the art. For example, drills for the installation of roof bolts in mines and the like have a hardened tungsten carbide blade mounted transversely on the distal end of an elongated drill shank. The body of the drill may also have access ports communicating with the interior of the bore for purposes of flowing water or applying a vacuum to remove dust and cuttings from the vicinity of the cutting action in the bore. The blades of such drills are adapted to bore a hole having a diameter of approximately one inch and larger into the hardened stone roof or earth strata of the walls of a mine.
In the distant past, it was common to forge a drill from hardened material or substance such that the distal end of the drill was shaped in a generally planar spade-like configuration with transverse cutting edges leading from a central point of the drill to the outer periphery of a cutting circle which the drill makes in the material or substance to be drilled.
An improvement of that basic structure has been proposed in the prior art in the form of attaching a spade drill blade in a slot at the distal end of a drill body by brazing or by some sort of a fastener. This permits the spade-like blade to be made of a hardened material or substance, while the drill body may be made of a softer, less expensive, material.
The blades of such drills are subjected to extreme forces causing stresses within the blade which frequently result in breakage of the blade and failure of the drill, and in particular, causes wear especially at the outer radial portions of the cutting edge of the blade insert. Such wear is caused by a number of factors, including improper alignment of the blade on the distal end of the drill body, excessive thrust being applied to the blade during the drilling operation, heat generated by the fact that the cutting edge of the spade insert is, at all time, in contact with the material or substance being drilled without any opportunity for cooling. Abrasion, frictional, and impact wear are also major causes of drill failure.
Attempts have been made in the past to achieve the goals of the present invention, but their efforts have fallen short of providing satisfactory results. For example, U.S. Pat. Nos. 5,287,937 and 5,458,210 to Sollami et al. show a drill with a blade insert having features which serve to centrally locate the cutting blade in the longitudinal recess of a drill body, but the cutting edges of the insert are of traditional shape and are thus subject to traditional wear and damage as described above.
Other examples of providing a spade blade insert into a receiving drill body can be found by reference to U.S. Pat. Nos. 4,086,972 to Hansen et al.; 4,817,742 to Whysong; 4,819,748 to Truscott; and 3,049,033 to M. L. Benjamin et al. While all of these prior art patents relate to spade drill insert arrangements, and while suggested improvements in blade cutting edge design and attachment means between the blade and the body of the drill are offered, none of these prior art references suggest any solution for the problem of wear of the cutting edges of a spade drill, especially toward the outer radial surfaces thereof.
U.S. Pat. No. 4,627,503 to Horton attempts to solve the wear problem by providing a multi-layer spade cutting insert comprising a polycrystalline diamond center layer portion and outer metal side portions. When used as an insert in a spade drill, the cutting element, while extending the life of the drill due to the presence of the polycrystalline material, the cutting edges must nevertheless be repeatedly resharpened, as mentioned in this prior art patent. Polycrystalline tool materials are very delicate and are very subject to impact chipping and breakage.
Attempts have also been made in the prior art to employ rotating discs to assist in the cutting action of a drill, examples being found in U.S. Pat. Nos. 1,692,919 to W. C. Bailey, and 1,790,613 and 1,812,475 to A. M. Gildersleeve et al. However, the rotary cutting discs as described in these prior art patents define the cutting edges of the drill devices themselves, i.e. they are not associated with any other drill cutting edges in combination.
It would be desirable to provide a rotary spade drill arrangement which would reduce cutting forces for the same rate of cut to thereby reduce the required thrust bearing forces, and to reduce the incidences of failure of the drill by extending the life of the drill several times over the life of a standard transverse edge spade drill arrangement.